A. Field of the Invention
This invention relates to a novel process for preparing dopes from which certain aromatic oxadiazole/N-alkylhydrazide copolymers may be isolated.
The dopes comprise a strong mineral acid (e.g. oleum) and a fiber-forming copolymer (hereinafter referred to as "Copolymer A") consisting essentially of recurring units (I) of the formula ##STR1## and units (II) of the formula ##STR2## in a mole ratio of between about 20:80 and 95:5, respectively, wherein .crclbar.X is an anion present in said acid, such as .crclbar.HSO.sub.4, .crclbar.HS.sub.2 O.sub.7, .crclbar.H.sub.2 PO.sub.4, .crclbar.H.sub.3 P.sub.2 O.sub.7, .crclbar.Cl and the like, Ar is an aromatic radical and R is an C.sub.1 to C.sub.4 alkyl. When the dope is combined with an aqueous medium, units (II) of Copolymer A hydrolyze to form units (III) of the formula ##STR3## and the resulting aromatic oxadiazole/N-alkylhydrazide copolymer (hereinafter referred to as "Copolymer B") precipitates or coagulates.
B. Description of the Prior Art
A process for preparing dopes from which shaped articles of Copolymer B can be made is described in British Pat. No. 1,407,439. The process involves mixing appropriate amounts of monomers consisting of an aromatic dicarboxylic acid, a dialkyl ester of an aromatic dicarboxylic acid and hydrazine sulfate in fuming sulfuric acid (oleum) at a temperature below which substantial polymerization of the monomers occurs, for example at 60.degree. C., until a solution of the monomers is formed. Dissolution of these monomers in oleum usually requires from 4 to 18 hours. The monomer solution is then heated at a temperature ranging from 125.degree. C. to 150.degree. C. for from 4 to 6 hours to polymerize the monomers and obtain a dope comprising Copolymer A in oleum.
The dope, after being diluted and degassed, may be extruded into an aqueous medium to provide fibers of Copolymer B by art-recognized solution spinning techniques, such as, by the technique described in British Pat. No. 1,417,568. The dope may also be used to cast films or provide coatings of Copolymer B.
The above-described process for preparing Copolymer A dopes has several serious drawbacks from the standpoint of commercial operations, for example, the process consumes considerable time and energy. Another disadvantage is that, even after the monomers and oleum are mixed at 60.degree. C. for several hours, undissolved monomer particles are nevertheless frequently present in the monomer solutions. When the monomer solutions are subsequently heated and the monomers polymerized, these particles cause gel formation. The presence of gel in the dopes tends to cause extrusion difficulties and product deficiencies.
An object of the present invention is to provide a process for preparing Copolymer A dopes which avoids the problems of the above-described process.
Other objects and advantages of the invention will become apparent from the following detailed description thereof.